1. Field of the Invention
The present invention relates to a pressure detection apparatus of an injection molding machine for detecting pressure by means of a load cell disposed between a movable member, such as a screw, and a drive mechanism.
2. Description of the Relevant Art
Japanese Patent Publication (kokoku) No. 8(1996)-2567 and Japanese Patent Application Laid-Open (kokai) No. 10(1998)-151653 disclose a conventional pressure detection apparatus of an injection molding machine for detecting pressure by means of a load cell disposed between a drive mechanism, which includes a drive motor and a ball screw mechanism, and a screw, which is driven in a reciprocating manner by the drive mechanism.
In such a pressure detection apparatus, one end face of a load cell is fixedly attached to a screw support member for supporting a screw, whereas the other end face of the load cell is fixedly attached to a ball screw or nut of a ball screw mechanism, thereby detecting pressure imposed between the screw support member and the ball screw or nut.
The above-mentioned pressure detection apparatus of an injection molding machine involves the following problems.
First, since the load cell is fixedly attached to both the screw support member and the ball screw mechanism by means of screws, the ball screw or nut imposes stress (load) directed in a rotational direction directly on the load cell, resulting in a failure to detect only thrust pressure at high accuracy.
Second, since the load cell also serves as a connection member for connecting the screw support member and the ball screw mechanism, impairment in the load cell (shortening of life of the load cell) is accelerated.
Third, since a value of detection by the load cell varies with, for example, temperature drift, reset-to-zero is usually performed while no pressure is imposed. However, since the load cell is fixedly attached to both the screw support member and the ball screw mechanism, imposed pressure cannot be released completely to zero, resulting in a failure to perform reset-to-zero accurately.
An object of the present invention is to provide a pressure detection apparatus of an injection molding machine capable of detecting only thrust pressure at high accuracy through prevention of imposition of stress (load) directed in a rotational direction on a load cell from a ball screw mechanism.
Another object of the present invention is to provide a pressure detection apparatus of an injection molding machine capable of lengthening life of a load cell through prevention of imposition of unnecessary stress on the load cell.
To achieve the above objects, the present invention provides a pressure detection apparatus of an injection molding machine comprising a drive mechanism, which in turn comprises a drive motor and a ball screw mechanism, and a movable member, such as a screw, which is moved in a reciprocating manner by the drive mechanism. The pressure detection apparatus comprises a pressure reception member disposed in the drive mechanism in such a manner as to be movable in a reciprocating direction of the movable member while being guided by a guide member, a ball screw or nut of the ball screw mechanism being fixedly attached to the pressure reception member; and a load cell disposed between the drive mechanism and the movable member in such a manner as to be fixedly attached to one of the movable member and the pressure reception member while abutting the other.
Thus, one of the movable member and the pressure reception member merely abuts the load cell, while the pressure reception member bears stress (load) directed in a rotational direction and imposed from the ball screw or nut of the ball screw mechanism. Thus, only thrust pressure is imposed on the load cell.